1. Field of the Invention
This invention relates to a stepless automatic transmission for motor vehicles, of the type employing a V-belt type stepless mechanism.
2. Description of the Prior Art
Conventional stepless automatic transmission employ a V-belt type stepless transmission mechanism including input and output pulleys, each with a variable effective diameter under control of a hydraulic servo, and a V-belt linking the input and output pulleys. Such a V-belt type stepless transmission mechanism is usually used in combination with a forward/reverse switch mechanism and a coupler such as a fluid coupling, centrifugal clutch or friction clutch. With this conventional type of V-belt type stepless transmission mechanism, in order to ensure quick and smooth increases and decreases in reduction ratio, it is necessary to hold at about 2 the load ratio of the input pulley to the output pulley. The approach which has thus far been resorted to for securing that load ratio is to make the effective pressure receiving area of a hydraulic servo of the input pulley about two times greater than that of the output pulley for the purpose of equalizing the operating pressures of the hydraulic servos of the input and output pulleys. However, the hydraulic servo of the input pulley, with such a large effective pressure receiving area, is found to be so bulky as to present problems with regard to mounting within the limited space available on the vehicle and will naturally invite a reduction in mileage due to its increased weight.